The invention relates to a method for loading a blank composed of fused silica with hydrogen. The invention also relates to a lens element composed of fused silica and to a projection lens, in particular for immersion lithography, comprising at least one such lens element.
Optical elements, e.g. lens elements, composed of fused silica are used for example in projection exposure apparatuses for microlithography at wavelengths in the UV range. Such lens elements are typically produced by mechanical processing of a blank composed of a synthetic fused silica. The synthetic fused silica of the blank is produced by combustion of a silicon-containing, organic or inorganic precursor substance in H2 and O2, if appropriate with addition of a fuel gas such as natural gas, for instance. SiO2 particles already form in the flame and are deposited on a target. In fused silica production, a distinction is made between the so-called direct process, in which a very hot flame is directed onto a hot target, such that the particles vitrify directly, and the so-called soot process, in which a porous body is deposited at lower temperatures and is sintered to form a solid glass body after drying and, if appropriate, doping.
Defects in the glass matrix of the fused silica, for instance missing bonds of the form O—Si. or O—Si—O, peroxy defects of the form Si—O—O—Si and ODC (“oxygen deficient centres”) of the form O—Si—Si—O or simply just weakened matrix bonds (“strained bonds”), exist to different extents after the production of the fused silica depending on the process control. The first two defects mentioned can additionally arise under laser irradiation during later use as an optical element as a result of the breaking of weakened matrix bonds or peroxy or ODC defects. All of the defects mentioned are distinguished by strong absorption bands in the UV wavelength range, and some even connected with laser-induced density changes such as “compaction”.
Absorption bands in the UV range are disadvantageous for the use of fused silica or of lens elements produced from fused silica in microlithography since radiation of a usually pulsed laser at an operating wavelength in the UV wavelength range is used there, to be precise typically radiation at a wavelength of approximately 193 nm (ArF laser). Hydrogen (or heavy hydrogen, i.e. deuterium) can saturate the first two defects mentioned to form O—Si—H or O—Si—OH (or to form O—Si-D or O—Si-DH), which have only weak absorption in the UV range.
In the case of directly deposited fused silica for use in the UV wavelength range, the flame stoichiometry is typically set such that there is a hydrogen excess and molecular hydrogen in the range of between approximately 5×1016 and 1019 molecules/cm3 is incorporated into the fused silica. At the same time, an OH content of between approximately 600 and 1300 ppm (by weight) is necessarily established in the fused silica material. Since the hydrogen is introduced hot during direct deposition, a high silane (SiH) content results. Such fused silicas are generally no longer used for lithography systems at an operating wavelength of approximately 193 nm, in particular for immersion lithography, since they exhibit strong polarization-induced birefringence and compaction (increase in density) during irradiation with higher energy densities and rarefaction (decrease in density) during irradiation with lower energy densities. With regard to the production of such a fused silica material, reference should be made for example to U.S. Pat. No. 7,265,070 B2, WO 02/29492 A1 and U.S. Pat. No. 7,064,093 B2.
Occasionally, instead of directly deposited fused silica, undried soot glass having an OH content of around approximately 250 ppm (by weight) or higher was used, which was sintered under inert gas (first generation) or hydrogen atmosphere (second generation) (cf. U.S. Pat. No. 7,082,790 B2 or DE 198 41 932 A1). Since the high OH content counteracts the formation of SiH, such comparatively dry glass could only be doped to significantly lower hydrogen contents than directly deposited fused silica.
In general, only soot glasses of the third generation are used nowadays for lithography systems operated at a wavelength of approximately 193 nm, which soot glasses are dried to OH contents of typically approximately 0.1 ppm to 100 ppm or 200 ppm, sintered, subjected to stress-relief annealing and only then loaded with hydrogen at “cold” temperatures of a maximum of 600°, but normally approximately 400° C. to 500° C. Such soot glasses exhibit compaction that is lower by a factor of 2-3 and FDT (“fluence dependent transmission”), polarization-induced birefringence and rarefaction that are lower by orders of magnitude in comparison with directly deposited glasses or soot glasses of the first or second generation; in this case, cf. for example: U.S. Pat. No. 7,501,367 B2, DE 10 2004 017 031 B4, U.S. Pat. Nos. 7,534,733 B2, 7,928,026 B2, 7,589,039 B2.
Cold loading, i.e. the loading of the fused silica blank with hydrogen at low temperatures, necessitates long loading times of a number of months for 5 to 10 cm thick blanks, depending on the required hydrogen content in the centre of the blank and the permissible hydrogen gradient in the volume of the blank. With a relatively long loading duration, more hydrogen diffuses into the centre of the blank, with the result that the gradient of the hydrogen spatial distribution in the blank decreases as the loading duration increases.
Although the diffusion time of the hydrogen into the fused silica material can be shortened somewhat by the use of an autoclave in which hydrogen is permitted to be kept at a pressure of more than 1 bar at the desired temperature, such an autoclave solves the problem of an excessively long loading duration only to a limited extent, since a hydrogen content in the edge regions of the blank of almost 1×1018 molecules/cm3 results even at a pressure of 2 bar, depending on the loading temperature used. If the required hydrogen content in the centre of the blank is an order of magnitude lower because hydrogen is no longer required there on account of the laser load and the loading time is intended to be kept as short as possible, this already results in a difference in refractive index at a wavelength of 193 nm of approximately 3 ppm between centre and edge of the (cylindrical) blank both along the cylinder axis and transversely with respect thereto. However, this difference in refractive index is of the same order of magnitude as, or is already greater than, the permissible refractive index inhomogeneity of a fused silica for lithography applications.
Two methods are known for nevertheless shortening the loading time: In a first method, which is described e.g. in U.S. Pat. No. 6,810,687 B2 and U.S. Pat. No. 7,994,083 B2, the partial pressure during loading is varied over time. The loading can start for example with a short loading at more than 100% hydrogen partial pressure relative to 1 bar ambient pressure, that is to say e.g. 100% hydrogen (H2) relative to 10 bar ambient pressure, corresponding to 1000% partial pressure relative to standard conditions. As a result, a very high hydrogen content is forced into the outer regions of the blank composed of fused silica. Afterwards, this quantity of hydrogen is distributed further into the blank by storage under standard pressure. If storage does not take place under a hydrogen atmosphere, some hydrogen is again diffused into the outer regions of the blank which are free of hydrogen after annealing with 0% partial pressure.
What U.S. Pat. No. 6,810,687 B2 and U.S. Pat. No. 7,994,083 B2 have in common is that all loading and annealing and/or discharge is performed at constant temperature. The aim of said documents is essentially to shorten the residence duration in the expensive pressure loading furnace or else, in the three-part method, the residence duration in the pressureless hydrogen furnace. The achievable shortening of the total throughput time compared with loading at constant pressure is of the order of magnitude of approximately 20% depending on the geometry of the blank and stipulations concerning hydrogen contents and permissible gradients. Moreover, in these methods the maximum of the hydrogen content does not occur at the surfaces of the blank, but rather further inwards, where the high hydrogen content is generally not required. In this regard, an increased SiH content occurs locally in the glass volume and four slopes of the change in refractive index are traversed instead of two in the blank in the z-direction (in the direction of the optical axis), which results in increased image aberrations.
A further method for shortening the loading time, described in DE 10 2007 022 881 A1, provides for firstly processing the blank mechanically to a geometry close to the final geometry and then loading it with hydrogen. Since the loading time of the blank, for the same hydrogen content in the centre of the blank and the same permissible hydrogen gradients, increases with the square of the thickness of the blank, the loading time for a typical meniscus lens element, the local thickness of which at all points corresponds approximately to half the height of the circumscribing cylinder, can be shortened by a factor of 4. However, this method provides only slight advantages for all other lens element shapes (biconvex, biconcave, planoconcave, planoconvex), since here the local thickness is only insignificantly less than the thickness of the original cylindrical blank from which the corresponding lens element is produced.
US 2011/0021339 A1 describes a method for producing a component composed of fused silica which is intended to have a saturated induced absorption at wavelengths of less than 250 nm. In order to achieve this, Si—O defects in the fused silica are first removed by silane (SiH) being formed at said defects. In order to form the silane, the fused silica is loaded with molecular hydrogen at temperatures of more than 475° C. After the silane has been formed, the fused silica is loaded with additional molecular hydrogen at high concentrations starting from 5×1016 molecules/cm3 at temperatures of less than 475° C., in order to support a conversion of E′ defect centres, which arise during the irradiation of the silane with laser radiation, into silane, i.e. the reaction equilibrium of SiH+photon<->Si.+H is intended to remain as much as possible on the left side.